[ARM] 5257/2: [AT91] Use SZ_ definitions and MTDPART_OFS_NXTBLK instead of hex-values

In the various AT91 board files, replace hard-coded size values (eg,
0x800000) with the SZ_ size definitions (eg, SZ_8M) from sizes.h
Also replace MTD partition offsets with MTDPART_OFS_NXTBLK.

Signed-off-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>